1. Field of the Invention
The present invention pertains to a method and apparatus for building and using look-up tables which determine the colors that a color printer prints in response to requests to print specific colors. The specific colors requested to be printed may include colors that are not printable by the printer. For those colors in particular, gamut mapping is performed in Munsell color space where hue planes are straight, i.e. not curved, so as to compensate for the Abney effect and to preserve the perceived hue.
2. Description of the Related Art
Recently, as the availability of color monitors and color printers has increased, it is more and more commonplace for a computer user to view a full color image on a color monitor and then to request a full color printout of that image on a color printer.
However, color printers and color monitors form color images differently. Specifically, a color monitor is a light emitting device; colors are formed on color monitors by adding light from three color primaries, generally, red, green and blue. Printed images, on the other hand, simply reflect ambient light; colors are perceived by the way ambient light is affected by three subtractive primaries, generally cyan, magenta and yellow (and sometimes black).
These processes are fundamentally different. As a result, the range of colors displayable on the monitor is different from the range of colors printable by a printer. FIG. 1 is the CIE 1931 chromaticity diagram showing the range (or "gamut") of colors displayable by a monitor (area "A") and the range (or "gamut") of colors printable by a printer (area "B"). As seen in FIG. 1, the range of colors displayable on a monitor is generally greater than the range of colors printable by a printer. This is because a monitor is a light emitting device and is able to display colors with greater saturation. There are, however, some low saturation areas such as at area 10 where a printed image, which is light-subtractive, has greater color range than a monitor.
Because of the difference between the ranges of printable and displayable colors, it has not heretofore been possible to print color images which are perceived as faithful reproductions of displayed color images. Specifically, it is simply not possible to print a color in areas like out-of-gamut area 11 which are outside the range "B" of printable colors. Accordingly, even though those colors may be seen on color monitors, they cannot be printed on a color printer.
In U.S. Pat. No. 4,941,038, out-of-gamut colors are adjusted to printable colors within the printer gamut by selecting the printable color which has the shortest vector distance from the unprintable color and which preserves the hue angle of the unprintable color. Experiments and observations on human color perception, however, have shown that a line representing a constant hue or color drawn from pure white out to the fully saturated color is not a straight line but rather is a curved line. The curvature of those lines (the so-called "Abney Effect") is shown in the chromaticity diagram of FIG. 2. For regions where the curvature of constant hue lines is relatively low, like bluish-green region 18, preserving hue angle only slightly changes the perceived hue. Thus, preserving hue angle for the bluish-green out-of-gamut color 18a by extending the hue back to point 18b on the printer gamut border 10 still prints a bluish-green color. But for regions where the curvature of constant color lines is relatively high, like purplish-blue region 19, preserving hue angle greatly affects perceived hue. Thus, preserving hue angle for the blue out-of-gamut color 19a by extending hue back to point 19b on the printer gamut border 10 prints a color with a decidedly purplish-blue hue.
Since each out-of-gamut color was adjusted independently, out-of-gamut colors are printed with poor color smoothness, where small changes in commanded color can result in large changes in printed color. In particular circumstances, poor color smoothness manifests itself as non-monotonic changes in luminance whereby the lightnesses of out-of-gamut colors does not increase smoothly and monotonically from dark to light but rather dips occasionally from light to dark. This results in a situation where colors which should merge smoothly and monotonically from dark to light in fact show undesirable bands of darkness.
In U.S. Pat. No. 5,299,291, which is assigned to the assignee of the present invention, to compensate for the Abney effect the hue angles in the printer table and border table are warped, and out-of-gamut colors are adjusted to printable colors within the printer gamut by building-printer tables which produce colors which vary smoothly in the out-of-gamut regions and exhibited monotonic increases in lightnesses. Transition colors are gamut mapped at a constant angle to colors of the same hue on the border of the printer gamut. However, since the gamut mapping is per, formed in CIELAB space, it is not easy to see which colors are of the same hue because the hue planes are slightly curved (the Abney effect). This can be seen in FIG. 3, where the gradation of colors of the same hue from white to a fully saturated color are shown projected into the a* b* plane at a Constant L*. The curved hue planes in CIELAB space are 5R (red), 5Y (yellow), 5G (green), 5B (blue) and 5P (purple), which are shown as curved lines. The warping computations in CIELAB space are inexact and time consuming. The contents of U.S. Pat. No. 5,299,291 are incorporated herein by reference.